In the packaging of certain types of foods, such as snack foods including candies, potato chips, cookies and the like, it is common practice to employ a multilayer film. Polypropylene films are widely used in the packaging industry due to their superior physical properties, such as, transparency, stiffness, moisture barrier characteristics and others. Despite these highly desirable properties, unmodified polypropylene film has the disadvantageous property of having a high inherent coefficient of friction and film-to-film destructive blocking on storage. This high film-to-film coefficient of friction makes polypropylene films difficult to be successfully employed in automatic packaging equipment in their unmodified form.
The bonding of metals, such as, aluminum, silver, chromium, etc., to plastic films and the gas barrier developed, has allowed such films to replace metallic foils in many instances. The flexibility of the films necessitates the formation of a strong metal/plastic bond and a number of approaches have been developed for providing such bonding. In some cases a special primer layer must be applied to the base layer in order to achieve an effective bond between the metal and the substrate. In many cases a thermoplastic surface must be corona discharged treated to provide an effective bond between the metal and the thermoplastic surface. Gas barrier properties will also depend upon the condition of the surface on which the metal is deposited.
Metallized oriented polypropylene (OPP) provides for an improvement in aesthetics due to the rich metal-like appearance of the package. Metallized OPP is formed by vacuum depositing a thin layer (100-600 Å thick) of aluminum onto the surface of a clear OPP base film substrate. The layer of aluminum that is deposited is particularly sensitive to damage such a scratching, pin-holing, pickoff, and metal crazing. Metal crazing occurs during extrusion lamination when the hot polyolefin from the die hits the aluminum surface of the metallized film causing it to crack in the cross machine direction. These cross machine direction cracks in the metal result in unacceptable aesthetics and, more importantly, adversely affect barrier properties. The cracks in the metal make it much easier for light, water vapor, oxygen and other gases to diffuse through the metallized film which results in poorer values for optical density, water vapor transmission rate (WVTR), and oxygen transmission rate (OTR). Metal crazing is also much more prevalent when the metallized film is run off the primary unwind of the extrusion laminator because of being subjected to more severe conditions compared to the secondary unwind. When the metallized film is run off the primary unwind, it typically has a longer web path prior to the laminating point and is also not against a chill roll at the point the polymer strikes the surface of the metal layer. Because the metallized film is not directly against a chill roll, there is a greater heat load on the film that results in a greater propensity for metal crazing. When run off the primary unwind, the metallized film is backed by a rubber coated nip roll at the lamination point. On the other hand, when the metallized film is run off a secondary unwind of an extrusion laminator, the web path is typically much shorter and the metallized film is backed by a large diameter water cooled chill drum when it is subjected to the hot melt. This results in much less of a tendency for metal crazing. Unfortunately, converters cannot always run the metallized film off the secondary unwind of the extrusion laminator, resulting in a metal crazed lamination unfit for commercial use.
U.S. Pat. No. 5,153,074 to Migliorini discloses a metallized film combination which includes a polymer substrate with an ethylene vinyl alcohol copolymer skin layer and a thin metal layer deposited on the surface of the ethylene vinyl alcohol copolymer. A maleic anhydride modified propylene homopolymer or copolymer is provided for adequate adhesion of the ethylene vinyl alcohol copolymer skin layer to the polymer substrate. The resulting metallized film exhibits outstanding resistance to the transmission of oxygen and other gases. U.S. Pat. No. 5,153,074 is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,194,318 to Migliorini et al. discloses a metallized oriented film combination which includes a propylene polymer substrate with a high density polyethylene skin layer and a thin metal layer deposited on the surface of the high density polyethylene. The resulting metallized film exhibits outstanding metal adhesion to the polymeric substrate and good barrier properties to moisture and oxygen. U.S. Pat. No. 5,194,318 is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,591,520 to Migliorini et al. discloses a metallized film combination which includes a propylene substrate with an amorphous polyamide skin layer and a thin metal layer deposited on the surface of the amorphous polyamide skin layer. A maleic anhydride modified propylene homopolymer or copolymer is provided for adhering the amorphous polyamide skin layer to the polymer substrate. The resulting metallized film exhibits outstanding resistance to the transmission of oxygen and other gases. U.S. Pat. No. 5,591,520 is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,753,363 to Bader et al. discloses a metallized film structure which includes an olefin substrate with a sealable skin layer comprising a propylene co- or terpolymer and a non-migratory particulate crosslinked hydrocarbyl substituted polysiloxane. The resulting metallized film exhibits low coefficient of friction and good machinability on packaging equipment. U.S. Pat. No. 5,753,363 is incorporated herein by reference in its entirety.
U.S. Pat. No. 6,086,982 to Peiffer et al. teaches a biaxially oriented polypropylene film of increased dimensional stability (shrinkage in the longitudinal direction and in the transverse direction of less than 5%). The film comprises a base ply having n-heptane-insoluble content of the film having a chain isotactic index of at least 97%. Such polypropylenes are made using metallocene catalysts. U.S. Pat. No. 6,086,982 is incorporated herein by reference in its entirety.
Objects and advantages of the present invention include one or more of the following:    To provide a metallized, multilayer polypropylene film having improved craze resistance in lamination applications.    To provide a metallized, multilayer polypropylene film having improved moisture and oxygen barrier after lamination.    To provide a metallized, white opaque multilayer polypropylene film having improved craze resistance in lamination applications.    To provide a metallized, white opaque multilayer polypropylene film having improved moisture and oxygen barrier after lamination.    To provide a metallized, multilayer polypropylene film that is more robust to laminating conditions, and in particular tensions, pressures and temperatures.    To provide a metallized film lamination including an extrusion laminated olefin layer adjacent to the metal layer that is free from metal crazing.    To provide a metallized film lamination including an extrusion laminated olefin layer adjacent to the metal layer that has improved moisture and oxygen barrier.    To provide a metallized white opaque film lamination including an extrusion laminated olefin layer adjacent to the metal layer that is free from metal crazing.    To provide a metallized white opaque film lamination including an extrusion laminated olefin layer adjacent to the metal layer that has improved moisture and oxygen barrier.    To provide a heat sealable, metallized, multilayer polypropylene film having outstanding machinability and sealability on high speed packaging lines.    To provide a heat sealable, metallized white opaque multilayer polypropylene film having outstanding machinability and sealability on high speed packaging lines.
This invention provides metallized multi-layer film having the above-mentioned desirable properties. This metallized multi-layer film comprises a core layer comprising high crystallinity polypropylene.